You can't really make a clear and simple rule is you allow for variables such as wind speed and direction therefore the clearest and simplest definition is to define the time limit for flying in still air conditions.

You can usually depart with an electrical generator U/S if you have the APU running throughout the flight.

Quoting smartt1982 (Thread starter):Is it ever a requirement to have the APU running for the entire ETOPS segment at all?

The answer is that it depends on the aircraft.
Some aircraft like B777 have enough generators, and a RAT, and can be dispatched with the APU inop.
Some aircraft like B737 have only 2 generators and no RAT and APU must be running.
Some aircraft like A320 have 2 generators and a RAT and the APU must be serviceable but not running.
And these requirements can change for 120mins and 180mins ETOPS.

Quoting Tristarsteve (Reply 2):Some aircraft like B737 have only 2 generators and no RAT and APU must be running.

As with almost everything, there are exceptions. On the 737NG, on a per airline basis, the FAA allows for the APU to not be running during ETOPS if certain maintenance, operational and reliability criteria are met.

Since it is not a safety critical thing (no APU before entering ETOPS airspace? Turn around to first alternate, less than 60min away), I guess it is acceptable. I would probably like to test it a bit more often though.

The light at the end of tunnel turn out to be a lighted sing saying NO EXIT

Its a limit set by the FAA, you would have to ask them. However, I had poorly written the requirement. The check has to be performed within 30 days from the previous check. This can be extended to 60 days or 90 days based on the airlines historical data of these checks and with FAA approval. I don't know what the criteria is to allow the extension.

There are other things that also have to be done to allow APU on Demand, such as a EGT monitoring program/limits, component requirements and a modified maintenance program.

Quoting Fabo (Reply 7):(no APU before entering ETOPS airspace? Turn around to first alternate, less than 60min away)

With APU on Demand, at no time during the flight including the ETOPS portion, does the APU need to be running.

Quoting yeelep (Reply 8):Its a limit set by the FAA, you would have to ask them. However, I had poorly written the requirement. The check has to be performed within 30 days from the previous check. This can be extended to 60 days or 90 days based on the airlines historical data of these checks and with FAA approval. I don't know what the criteria is to allow the extension.

There are other things that also have to be done to allow APU on Demand, such as a EGT monitoring program/limits, component requirements and a modified maintenance program.

The CMP requires 30 days, but can be extended to 120 days. The criteria required to allow the extension is proven reliability of the program, including the 95% start success rate.

Quoting yeelep (Reply 10):As its a FAA approved program, only US based Airlines that fly 737NG's to Hawaii can benefit.

Other governing agencies follow the FAA requirements, so non US carriers would also benefit. There are currently 2 US operators using APUOD, and another operator across the pond is seeking the program.